How to Read a Tire Size

What does P mean in a tire size like P225/45R17?

P stands for Passenger — it identifies the tire as built to TRA (Tire & Rim Association) P-metric standards for passenger cars, minivans, and crossovers. A tire without the P prefix (e.g., just 225/45R17) is Euro-metric, built to ETRTO standards. The geometry is the same, but the load tables differ slightly. When a P-metric tire is used on a light truck or SUV, its load capacity must be derated by approximately 10% per TRA tables.

What is the aspect ratio on a tire?

The aspect ratio is the second number in a tire size — it describes the sidewall height as a percentage of the section width. In 225/45R17, the aspect ratio is 45, meaning the sidewall height is 45% of 225 mm = 101.25 mm (about 4 inches). A lower aspect ratio means a shorter sidewall (low-profile tire), which improves steering response and reduces sidewall flex but reduces ride cushioning. A higher aspect ratio means more sidewall height, more cushioning, and better off-road compliance.

How do I calculate overall tire diameter?

Use these formulas: Sidewall height (mm) = Section Width × (Aspect Ratio ÷ 100). Overall diameter (mm) = (2 × Sidewall Height) + (Rim Diameter in inches × 25.4). Convert to inches: divide by 25.4. Example: 225/45R17 → Sidewall = 225 × 0.45 = 101.25 mm → Diameter = (2 × 101.25) + (17 × 25.4) = 202.5 + 431.8 = 634.3 mm = 24.97 inches.

What does XL mean on a tire?

XL stands for Extra Load — not extra large. An XL tire has a reinforced construction that allows it to be inflated to a higher reference pressure (typically 42 psi vs 35–36 psi for a Standard Load tire of the same size). This allows it to carry a higher maximum load, which is why XL tires often have a higher load index (e.g., 94W XL vs 91W SL for the same 225/45R17). The external dimensions are the same — only the load capacity and recommended pressures differ.

Can I use Euro-metric tires instead of P-metric?

For passenger cars, a Euro-metric tire of the same size (e.g., 225/60R16 instead of P225/60R16) is generally considered interchangeable if the load index and speed rating match or exceed OEM specifications. The geometry is identical. The difference matters most for light trucks and SUVs: P-metric tires on these vehicles carry a 10% derating under TRA standards. If you match load index and speed rating on a passenger car, the system difference is not practically significant.

What is the 3% tire size rule?

The 3% rule is an industry convention (not an NHTSA regulation) stating that replacement tires should not differ more than ±3% from the OE overall diameter. Staying within 3% keeps speedometer and odometer error within acceptable bounds, minimizes ABS/ESC calibration drift, and avoids clearance issues. Exceeding 3% without recalibrating vehicle systems can cause ABS and stability control to malfunction, especially if only one axle is changed.

What is a flotation tire size like 33x12.50R15?

Flotation sizes (also called "inch sizes") are used primarily for off-road and light truck tires. The format is: Overall Diameter × Section Width R Rim Diameter. In 33x12.50R15LT: 33 = 33 inches tall (overall diameter), 12.50 = 12.5 inches wide (section width), R = radial, 15 = 15-inch rim, LT = light truck construction. The 33x12.50R15 is approximately equivalent in geometry to a 315/75R15 in P-metric notation.

How does changing tire size affect speedometer readings?

Speedometers count wheel revolutions and multiply by the expected circumference. If you install tires with a larger diameter, each revolution covers more ground than the vehicle expects — so the speedometer reads lower than your actual speed. Formula: Actual Speed = (New Diameter ÷ Original Diameter) × Indicated Speed. A 10% larger diameter means you are actually traveling 10% faster than the speedometer shows. At 60 mph indicated, you are actually doing 66 mph.

What is plus sizing?

Plus sizing means increasing wheel diameter by 1–3 inches while reducing the tire's aspect ratio to keep overall diameter approximately the same. Example: going from 225/50R17 (OE) to 225/40R18 keeps the overall diameter within ~2% while fitting an 18-inch wheel. The benefit is a larger brake rotor space and a sportier appearance. The trade-off is a shorter sidewall, which reduces ride cushioning and increases vulnerability to pothole damage. Calculating the correct aspect ratio for the new rim size: H_new = (Original Diameter mm − (New Rim in inches × 25.4)) ÷ 2.

What load index should my tires have?

Your replacement tires must meet or exceed the load index specified for your vehicle — found on the door jamb sticker. For passenger cars, use the exact OE load index or higher. For light trucks and SUVs with P-metric tires, the load index must account for the 10% derating: the P-metric load rating × 4 tires ÷ 1.10 must equal or exceed GVWR. For vehicles originally equipped with LT-metric tires, replacement tires must be LT-metric with equal or higher load index and load range. Downgrading load index, especially on trucks or when towing, is a safety risk.